1. Field of the Invention
This invention relates to cathodes for electron tubes such as cathode-ray tubes of TV sets and particularly to an improvement in electron emission characteristics of an oxide-coated cathode.
2. Description of the Background Art
FIG. 1 is a sectional view schematically showing a conventional oxide-coated cathode for used in a cathode-ray tube or an image pickup tube for a TV system. In the conventional oxide-coated cathode, an electron-emissive substance layer 2 made of alkaline earth metal oxides containing at least Ba and further containing Sr and/or Ca is formed on a cylindrical base 1 made of Ni as a major element containing a small amount of a reducing element such as Si or Mg. A heater 3 is provided inside the base 1 and the electron-emissive layer 2 is heated by the heater 3 to emit thermal electrons. At this time, main donors for the emission of thermal electrons are free Ba reduced by Si, Mg or the like.
Such a conventional cathode is manufactured by a process as described below. First, a suspension of carbonates of alkaline earth metals (Ba, Sr, Ca, etc.) is applied on the base 1 and heated in vacuum by the heater 3. As a result, the alkaline earth metal carbonates are converted to oxides. Then, the alkaline earth metal oxides are partially reduced at a high temperature of 900.degree. to 1100.degree. C. so that they are activated to have a semiconductive property, whereby the electron-emissive layer 2 made of alkaline earth metal oxide is formed on the base 1.
In the above described activation process, reducing elements such as Si and Mg contained in the base 1 diffuse to move toward the interface between the alkaline earth metal oxide layer 2 and the base 1, and then react with the alkaline earth metal oxides. For example, if the alkaline earth metal oxide is barium oxide (BaO), the reaction is expressed by the following formula (1) or (2). EQU BaO+1/2Si=Ba+1/2SiO.sub.2 ( 1) EQU BaO+Mg=Ba+MgO (2)
Thus, the alkaline earth metal oxide layer 2 formed on the base 1 is partially reduced to become a semiconductor of an oxygen deficient type. Consequently, an emission current of 0.5 to 0.8 A/cm.sup.2 is obtained under the normal condition at an operation temperature of 700.degree. to 800.degree. C. However, in the cathode thus formed, a current density higher than 0.5 to 0.8 A/cm.sup.2 can not be obtained for the following reasons. As a result of the partial reduction of the alkaline earth metal oxides, an interface layer of oxides or composite oxides such as SiO.sub.2, MgO, and BaO.SiO.sub.2 is formed in the interface region between the base 1 and the alkaline earth metal oxide layer 2 as is obvious from the formulas (1) and (2). Particularly, the interface layer tends to be formed at nickel crystal grain boundaries near the interface region and at a position of about 10 .mu.m from the interface into the electron-emissive layer 2. This interface layer is a layer of a high resistance which obstructs flow of current. In addition, it is believed that the interface layer prevents the reducing element in the base 1 from diffusing into the electron-emissive layer 2, and thus, prevents formation of a sufficient amount of Ba for emitting thermal electrons.
Japanese Patent Application No. 229303/1985 discloses a cathode comprising a base 1 of Ni containing a rare earth metal of 0.1 to 0.5 wt. %. In this cathode, oxidation of the base 1 is prevented when alkaline earth metal carbonates are decomposed to form the electron-emissive layer 2 or when barium oxide is reduced during operation of the cathode. In addition, an interface layer of composite oxides is prevented from being formed in a concentrated manner near the interface between the base 1 and the electron-emissive layer 2, and the composite oxides is formed in a diffused manner in the electron-emissive layer 1. Accordingly, a moderate diffusion of the reducing element such as Si or Mg is maintained. As a result, there is less deterioration of the electron emission characteristics in operation of the cathode even at a high current density of about 1 to 2 A/cm.sup.2.
Japanese Patent Application No. 160851/1985 discloses a cathode comprising an electron-emissive layer 2 containing a rare earth metal oxide of 0.1 to 20 wt. %. Also in this cathode, oxidation of the base 1 is prevented and formation of an interface layer is prevented. The electron emission characteristics of this cathode are little deteriorated in operation even at a high current density of 2A/cm.sup.2 as in the above mentioned cathode. However, a further improvement is still required. More specifically, if the cathode after the normal activation process is operated at a high current density of more than 2A/cm.sup.2, it happens that free Ba is considerably evaporated to deteriorate the electron emission characteristics.